1. Field of the Invention
This invention relates in general to a sheet ejecting device, a curl eliminating device and an image forming apparatus, and more specifically relates to a curl straightening structure to straighten the curl created on the sheet.
2. Description of Related Art
Regarding to an image forming apparatus such as a copy machine, a printer, a facsimile or the like, after an electrostatic latent image is formed onto a latent-image-supporter, the electrostatic latent image is then treated in an image visualization of the developing process. The visible image is electrostatically transcribed onto a recording paper which is a recording medium. Then the transcribed visible image is for example heated to fix to obtain a copy or record thing.
A heat roller and a pressure roller, which face-to-face clamps to form a sheet-transporting-passage, are disposed on one of the devices that are used for fixing, and during transportation of the sheet by the tow rollers, a toner image is fixed by the heat roller with heat and pressure.
The fixing of the heat roller is widely used nowadays due to its advantages such as achieving high efficiency and high speed of the heat roller, achieving high conductivity and stability in fixing efficiency, and having simpler structures that are capable of using a sheet-transporting-medium.
When the fixed sheet is ejected to an ejecting tray by ejecting rollers that are disposed near the heat roller and the pressure roller, the sheet will be re-transported to face the resist roller, rather than to face the ejecting tray. Alternatively, when the sheet is being re-transported and reversed to pass the fixing device, the back side of the image formed face can be selected to be re-formed and then to be ejected to the ejecting tray.
The sheet, when passing the fixing device, will expand due to the heat supplied when the sheet passes the rollers installed inside the fixing device, and will contract due to the evaporation of water caused by the heat. At this time a stress range is created and a deformation called curling happens. As a result when ejecting a sheet with curling, the tip of the sheet will hang downward and then a jam is created by the tip protruding and hitting a transporting face of the ejecting tray such the transportation of sheets thereof is obstructed.
The amount curling is proportional to the amount of the toner supplied onto the sheet. For example, the amount of the toner supplied onto the sheet is larger for a full color image than that for a monochrome image. In other words, the expansion/contraction is relatively obvious when the full color image is formed.
Though curling can also be created due to the manufacture of the sheet at the very beginning, curling is generally created when fixing the sheet. Because the inside of the sheet will become crooked due to the transmutation of the sheet when fixing the sheet, and despite each sheet having a similar tendency to crook, the curling in each sheet differs from each other.
If the curling is kept on the sheet, the toner image and the transcribing position of the sheet will not match due to the curling, and then the sheet will hit the parts arranged on the transporting passage causing jams. Furthermore, the consistency of the ends of the ejected sheets (the stacking quality) will be reduced or inconvenience will occur.
In order to straighten the curling of a sheet, some conventional structures are developed as follows: one structure is made in a way that the ejecting rollers, which clamp and transport a sheet, are made face-to-face and the curvature radius of one of the ejecting rollers is extremely smaller than that of the other one, then with the difference in curvature, the sheet will be bent in a direction reverse to the direction of the curl during transporting (such as Japan Laid-Open publication no. He5-162916); another structure is made in a way that, one side of the transporting guide unit is extended in a direction reverse to the direction of the curl to allow the sheet to conform with the shape of the side of the transporting guide unit to straighten the curl during which it moves from the side of the transporting guide unit to the other side within the transporting passage; still another structure is made in a way that eliminating the curl with an orientation so the sheet moves along the guide face of the transporting guide; still another structure is made in a way that to change the position where the sheet is clamped and transported in, and to change the transporting direction in order to straighten the curl (such as Japan Laid-Open publication no. He7-121039, He7-285721, He8-137309 and He8-290857); still another structure in made in a way to straighten the curl by using the circumference of the ejecting rollers by making a pair of additional rollers contact along the circumferences of the ejecting rollers and clamping and transporting the sheet by the ejecting rollers and the pair of additional rollers, so that the curl of the sheet can be straightened along the circumferences of the ejecting rollers (such as Japanese Patent no. 2547722 and Japan Laid-open publication no. He11-189363).
The temperature of the sheet will rise during heating after passing the fixing device. The temperature of the units constructing the transporting passage or the units arranged on the fringe of the fixing device will tend to rise also. The problems then surface due to the sheet deformation or damage due to the temperature rising and the sheet contamination resulting from the toner supplied onto the surface of the sheet, because the toner is remelted and fallen down. As a result, a sheet with low viscosity is adhered to the transporting unit. When the ambient temperature rises caused by the temperature of the sheet rising after the sheet passed the fixing device, the inner temperature of the other nearby devices arranged near the fixing device such as the cleaning device or the developing device will also rise affectedly. At this time, the collected sheet will possibly be remelted by the cleaning device, and the sheet received inside the developing device will possibly be transmuted.
Additionally, the curling can also be affected by the shape of the transporting passage when passing the fixing device to the curl straightening device due to the heat accumulation when the sheet is heated by the fixing device. For example, in a case when the transporting passage from the fixing device to the curl straightening device is steep and winding, the sheet will easily curl according to the shape of the transporting passage due to the ironing effect from the heat accumulation of the sheet. It can be easily thought as being similar to pressing by the iron to stretch wrinkles.
This kind of transporting passage, which demands more developing units than forming the monochrome images, is widely used to form color images aiming to prevent a large-size structure and to bend the transporting passage to minimize the space in transverse direction or in lengthwise direction. This kind of the transporting passage can also be used to reverse the sheet to form two-face images.
In the conventional structures, in order to get rid of the damages due to the rising temperature, it is proposed to use a transporting belt connected to a portion of the heat pipe to transport the sheet in the transporting passage (such as the Japan Laid-open publication no He11-119489).
The face of a sheet, which supports the toner image, bends towards a side and causes curling resulting form the difference of the humidity between the face of the sheet supporting the toner and the face not supporting the toner. Therefore, the conventional curl straightening structure eliminates the curl by loading in a direction reverse to the direction of the curl to turn the sheet back.
Among the structures that bend the sheet in a direction reverse to the direction of the curl, the ones using a non-moving guide unit to construct a transporting passage cause the tip of the sheet to make contact with the guide unit again after it has been straightened during moving and therefore the tip of the sheet bent reverse to the curl will easily encounter a large resistance during movement. Consequently, failures will occur during transportation.
For solving the problem, the Japanese Patent no. 547722 and the Japan Laid-open publication no. He11-189363 disclose that in the case when transporting rollers are used in the sheet transporting passage to straighten curling, it is necessary to keep the transporting rollers in contact with the ejecting rollers, and it is further necessary to lead the sheet towards the contacting position of the ejecting rollers and the transporting rollers, therefore the structures are complicated. Especially, when the sheet is entering the contacting position on the ejecting rollers and the transporting rollers, the sheet is separated from the transporting guide unit, therefore it is difficult to lead the sheet to the contacting position. Consequently some failures will easily occur during transportation which is similar to the case described above.
The conventional structures for straightening curling using other methods are as follows: one structure is disclosed in the Japan Laid-open publication no. He11-189363, wherein a heat-resistant material is used for constructing the offset preventing layer with fluorine coated onto the surface thereof to form the structure with low heat-conductivity. Therefore, heat from the sheet is easily accumulated. When the sheet is continuously passed through the fixing device, the heat accumulated will increase on the rollers at the side in contact with the toner. In other words, the surface temperature of the roller will rise and the rollers will serve to heat up other rollers in contact thereon.
Under this condition, the sheet will pass through the fixing device and will be heated by the opposite rollers in contact thereon. Then the sheet will deteriorate and the toner on the fixed sheet may remelt due to the rising temperature and cause offset.
However, in the structure described in the publications above, if the structures of curl straightening and cooling are separately constructed to form the structure with individual functions, it may cause the structure to be a large-size or may increase the cost.
For solving the problems of the conventional ejecting structures, the present invention proposes an ejecting device and an image forming apparatus to prevent the structures from being large in size, to prevent the parts from deteriorating due to heat, to efficiently straighten the curl, and to restrict the temperature rising in the curl straightening unit to restrict offset.
For solving the problems of the conventional sheet transporting device, especially for the problems related to the curl straightening of the sheet, the present invention propose a curl eliminating device of the sheet and an image forming apparatus to cool and straighten curling of the sheet at the same time without increasing the cost.
The present invention proposes a sheet ejecting device using a heat member and a press member arranged opposite to each other within a transporting passage of a sheet which supports a toner image to fix the sheet by heating and pressing and afterwards ejecting the fixed sheet. The sheet ejecting device further comprises a de-curl mechanism, including: a first roller positioned at a side facing and contacting the toner image supported by the sheet; a plurality of second rollers arranged abreast on a circumference of the first roller and contacting the circumference of the first roller. The second rollers clamp and transport the sheet to allow the sheet to conform with the circumference of the first roller. The de-curl mechanism bends the sheet reverse to a curl-generating direction of the sheet to straighten a curl. One of the rollers constructing the de-curl mechanism is made of a cooling member.
In the present invention, the cooling member is used to the first roller positioned at the side facing and contacting the toner image.
In the present invention, the cooling member is a roller bending the sheet reverse to the curl-generating direction.
In the present invention, the cooling member is made by a heat pipe.
In the present invention, a material, serving as a base of the heat pipe, is used directly in contact with the sheet to cool the sheet.
In the present invention, the heat pipe comprises a radiating fan arranged on an end of an axial direction of the heat pipe, and the radiating fan is cooled by a cooling fan arranged near the radiating fan.
In the present invention, the cooling fan is capable of controlling an airflow according to a temperature of the heat pipe.
In the present invention, the radiating fan of the heat pipe is connected to a sheet dehumidifying unit.
In the present invention, a tension of the sheet, passing the de-curl mechanism, is changed at an entrance side and an exit side of the de-curl mechanism.
In the present invention, the tension of the sheet is larger at the exit side of the de-curl mechanism than that at the entrance side.
In the present invention, among the second rollers, a rotating speed of the second roller positioned at a downstream side in an ejecting direction of the sheet is larger than that of the second roller positioned at an upstream side.
In the present invention, among the second rollers, an outer diameter of the second roller positioned at the downstream side in the ejecting direction is large than that of the second roller positioned at the upstream side.
The present invention further proposes an image forming apparatus comprising one of the sheet ejecting devices described above.
In the present invention, the de-curl mechanism is arranged before the sheet, transported after being fixed, is led into a sheet reversing passage.
The present invention proposes a curl eliminating device, comprising: a transporting roller, arranged in a sheet transporting passage; and a plurality of backup rollers, arranged abreast along a circumference of the transporting roller. The backup rollers press-contact the transporting roller during rotating. The curl eliminating device clamps and transports the sheet in a way that the sheet conforms with the shape of the circumference of the transporting roller. The backup rollers are separately arranged along the circumference of the transporting roller, and a separating distance is set to be changeable. The separating distance is set according to the amount of curling of the sheet.
In the present invention, the backup roller further comprises a plurality of roller units subdivided in axial direction of the backup rollers. The roller units are arranged in the axial direction with position-shift and an operation timing of each backup roller can be changed.
In the present invention, the transporting roller is used to as a sheet cooling member, and the friction between the transporting roller and the sheet is set smaller than that between the backup rollers and the sheet.
In the present invention, relative circumferential-speeds of the transporting roller and the backup rollers can be changed.
The present invention further proposes an image forming apparatus, comprising one of the curl eliminating devices described above, and with using the curl eliminating device, the transporting roller can cool the sheet.
The present invention further proposes an image forming apparatus, comprising: a fixing device using a heat member and a press member arranged opposite to each other within a transporting passage of a sheet which supports a toner image to fix the sheet by heating and pressing and then ejecting the fixed sheet; a reversing device for reversing and transporting the sheet ejected from the fixing device; a transporting roller, having a promptly cooling structure to construct a cooling member to cool the sheet during transporting, is arranged in the transporting passage, going towards the reversing device from the fixing device; and a plurality of backup rollers arranged abreast on a circumference of the transporting roller for clamping and transporting the sheet in a way that the sheet imitating a shape of the circumference of the transporting roller. The transporting roller and the backup roller are arranged in a way that can bend the sheet oppositely to a curl-generating direction of the sheet, when it is ejected from the fixing unit, and then to move the sheet toward the reversing device.